This invention relates in general to television receivers and in particular to a field identification system used in a digital television receiver.
The Advanced Television System (ATV) recently adopted by the FCC for use in the United States for both high definition television (HDTV) and lower resolution HDTV signals incorporates vestigial sideband (VSB) transmission. The VSB transmission system is characterized in both the terrestrial and cable modes by a data frame that comprises two data fields of 313 data segments each with each data segment comprising 832 multilevel symbols. The first data segment in each field comprises a data field sync segment and each data segment is headed by a four symbol data segment sync followed by 828 data and forward error correction symbols. (At present, the data field sync segments do not include forward error correction symbols.) The field and segment sync symbols facilitate recovery of the data in the ATV receiver and provide timing signals for the fields and segments. Each field sync segment also includes information identifying the VSB mode.
The transmission is via suppressed carrier modulation. Three hundred and ten kilohertz from the lower band edge, a small DC pilot is added to the signal for use by the VSB receiver in achieving carrier lock. The data and forward error correction (FEC) bytes are randomized and interleaved for added protection against burst errors.
The symbol rate is approximately 10.76 MHz. The sync symbols are always two level. In the terrestrial mode, 8 level trellis-coded symbols (3 bits per symbol) are transmitted, whereas in the cable mode, 16/8/4/2 level symbols (4/3/2/1 bits per symbol, respectively) are used.
As mentioned, each field is headed by a data field sync segment. As is fully described in the above-referenced copending application, the data field sync segment is characterized by a relatively long (511 symbol) pseudo-random number sequence followed by three relatively short (63 symbol) pseudo-random number sequences with the middle one of the short pseudo-random number sequences alternating in polarity in successive fields. Both the relatively long pseudo-random number sequence and the three relatively short pseudo-random number sequences are used as an equalizer training signal in terrestrial broadcast applications and one of the relatively short pseudo-random number sequences is used as an equalizer training signal in more benign applications, such as in a cable environment.
In some applications it is advantageous to be able to identify the first field in a frame. The presence of the reverse polarity middle 63 pseudo-random number (PN) sequence is used in the present invention for that purpose.